Spark plug and method of making same



June 15, 1943. 'r. G. M DOUGAL SPARK PLUG AND METHOD OF MAKING SAME Filed Jan. 23, 1942 2 my m 6 J 2 5 ,z 4 J j H Inventor ZZZ/76 6 ZZZY'QWga Z 1 Wigs .4 llllllllllllllllllllll I! Patented June 1 5, 1 943 SPARK PLUG AND ME SAM THOD OF MAKING E Taine G. McDougal, Flint, Mich, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application January 23, 1942, Serial No. 427,867

13 Claims.

This invention relates to spark plugs and their manufacture, particularly to the glazing of spark plug insulators and assembling the electrode therein.

In the manufacture of spark plugs it has been customary to glaze that portion of the insulator which is exposed above the metal housing. This exposed surface becomes coated with dirt, grease and the like in service, which may result in the formation of an electrically conducting path, causing the spark plug to fail because of a flashover along the insulator surface. When the surface thus exposed is glazed it is smooth, and dirt, grease and other accumulations can be easily wiped off. Furthermore, the dirt, etc. does not adhere so readily to such a surface in the first place.

In ordinary one-fire spark plug insulators the glaze may be sprayed on the unfired insulator, after Which the glaze and insulator body may both be matured in the same firing or heat treatment. Such a glaze coating does not appreciably affect the dielectric characteristic of the insulator. However, with the insulators in use in modern plugs such as the high alumina insulators which require a high temperature for firing of the ceramic material it is necessary in most instances to employ a secondary firing operation to mature the glaze. The reason for this is that the sintered oxide insulators, such as alumina, must be fired to such high temperatures to mature them to a dense and nonporous structure that any glaze applied before firing would probably be volatilized and/or be absorbed into the insulator structure. Sintered oxide insulators such as alumina are fired at high temperatures of the order of magnitude of Orton cone 23 i 44, whereas ordinary one-fire porcelain type insulators are fired to temperatures of the order of magnitude of Orton cone 15 to 20; a. wide range of silicate glaze compositions is available for use at the lower temperatures. If once-fired high alumina insulators are coated with a glaze subsequently matured at Orton cone 15 to 20 there is a substantial lowering of the insulators high temperature electrical resistance, known in the art as the Te value. This lowered Te value can be attributed to the absorption of alkali and other electrically conducting ions from the kiln atmosphere and possibly from the glaze.

It is the object of this invention to provide a spark plug and method of producing same, in which a ceramic insulator which has been fired to a high temperature is coated with a glaze which can be matured without impairing the high temperature electrical resistance of the insulator. The insulator body is preheated to a temperature which need be merely high enough to dry the glaze as soon as it is applied. If desired, a trade-mark may then be sprayed over the glaze. The trade-mark will also dry on the warm insulator. The insulator can then be fired to the low maturing temperature of the glaze without injury to the dielectric value of the insulator.

Where a composite conductor element is used, in which the parts are joined by a conducting seal, it has further been found that the sealing material can be softened at the same time that the glaze is matured. Where such a conductor element is used the insulator is first sprayed and is then assembled loose with its electrode, sealing material and terminal. This loose assembly may then be passed through a furnace to heat the sealing material to its oftening temperature at the same time that the glaze matures. After the assembly leaves the furnace, pressure may be applied, if necessary, to force the sealing material into place.

Figure 1 is a view showing the glaze material being sprayed on that portion of the insulator which is to be exposed.

Figure 2 shows the trade-mark being applied.

Figure 3 is a view in longitudinal section showing insulator, composite electrode and sealing material before final assembly; and

Figure 4 is a view in longitudinal section of a completed spark plug made according to this invention.

Referring more particularly to the drawing, especially to Figure 1, 2 is a preheated spark plug insulator mounted on any suitable support 4 to which a glaze material 6 is applied by means of any suitable nozzle 8; apron I0 may be provided if desired to shield the lower portions of the insulator from the spray of glazing material. Insulator 2 is preferably preheated to cause the glaze material 6 to dry upon application. A temperature in the vicinity of C. to C. has been found satisfactory for the preheat, al-

though it should be understood that any other temperature which sufiices to dry the glaze material on application will be satisfactory. After the glaze material has been applied, as in Figure 1, and while the insulator still retains some of its preheat, a trade-mark or other distinctive marking may be applied as shown in Figure 2. As in Figure 1, insulator 2 is carried ona support 4 and the mark forming material G'is applied as by spraying from a suitable nozzle 8'. Shields =iowtemperature I mean here a temperature the order of Orton cone 010 to 02. The following composition is an example of a glaze which has the desired characteristics:-

Per cent Frit A 31.8 Frit B 59.1 Clay 6.4 Opax 2.7

Frits A and B may have the following composition:

To this composition may be added flocculating agents to obtain the viscosity necessary for spraying the slip where the glaze is applied by spraying. These agents are added in the mill charge together with the water to form the slip. The powdered material making up the glaze is ground to a fineness of less than 0.5% on a screen having 325 meshes to the inch. The slip should have a specific gravity between 1.64 and 1.66.

After the glaze has been sprayed on and the trade-mark has been applied, as shown in Figures 1 and 2, the insulator is provided with the conductor element components as shown in Figure 3. These components may comprise an electrode indicated generally by IS, a terminal l8 and a conducting seal which may, if desired, be made up of two sealing materials 20 and 22, as disclosed and claimed in U. S. Patent 2,248,415 to Schwartzwalder et a1. Sealing materials 20 and Marc preferably mixtures of glass and a finely divided electrical conductor. The material 22 preferably has greater plasticity than material 20. The glass of material 20 may conveniently be a hard glass such as a borosilicate glass, known as Pyrex; the glass of material 22'may be a soft glass, such as lead borosilicate glass. For details of the composition of materials 20 and 22, reference may be had to the above mentioned patent to Schwartzwalder et al. Electrode l6 may be a single piece of any suitable heat resisting alloy or it may, if desired, be made of a heat resistant shell with a core having high heat conductivity, as shown in the drawing. A heat resistantshell 24 is provided with a core 26 of any suitable sub stance having high heat conductivity such as copper. Core 26 is bonded to shell 24 by any suitable means such as silver solder, a pellet of which is shown at 28 in the bottom of shell 24.

In operation, the steps in my improved manufacturing process are the following: an insulator which has been fired to a high temperature and mango-av which has thereafter been preheated to a temperature in the vicinity of 100 C. is coated with a glaze forming material and a marking material as shown in Figures 1 and 2. The insulator with its dried coating of glaze forming material is then provided with an electrode, as shown in Figure 3, which is placed in the longitudinal bore 30 of the insulator and at the firing end thereof. Sealing materials 20 and 22 are placed in the longitudinal bore adjacent the electrode. The material may be in the form of pellets as shown at 22 or either or both of these materials may be provided in the form of a powder as shown at 20. The preferred method is to take a pellet such as is indicated at 22 and crush the same after it has been dropped into place in bore 30. The resultant granular mass is" the powder which is shown at 20. Pellett 22 is then placed into bore 30 adjacent sealing material 20. If desired, of,course, only one type of sealing material need be used. The terminal is then placed into the bore 30 adjacent the sealing material. The resultant. loose assembly consisting of insulator, electrode, sealing material, and terminal, is then fired in a furnace to a temperature sufilcient to soften the pellet of solder 28 and the sealing material 20 and 22, and to mature the glaze form ing material. It will generally be found expedient to apply pressure to terminal IS with insulator 2 held in any suitable support. Such pressure should be sufiicient to cause solder 28 to flow into position, forming the bond shown at 28' in Figure 4. This pressure will also force terminal 18 down through the sealing material 22 so that material 22 occupies the position substantially as shown at 22' in Figure 4. It will be understood, of course, that the granular mass 20 is converted into the vitreous mass 20' shown in Figure 4. As is explained in the above patent 40 to Schwartzwalder et al., sealing material 22 is more plastic than material 20 and is more easily made to fiow about the lower end of terminal Hi. If desired, terminal l8 may be provided with surface irregularities as shown at 32 in order 'to give the sealing matter 22 a better gripping action for increased mechanical strength. To this end, also, longitudinal bore 30 may be provided with irregularities as shown at 34.- Electrode l6 may be provided with a slot 36 for better gripping action of sealing material 20. It will be understood that in the preferred form terminal I8 will come into contact with conducting seal 20' in the final assembled condition as shown in Figure 4. However, if thi contact is not made it will not result in inoperativeness inasmuch as seal 22 will likewise be made conducting. The insulator and its conductor element are then assembled in a conductor shell 38 as seen in Figure 4.

0 I claim:

1. In spark plug manufacture, the steps of coating an insulator with a glaze forming material, putting an electrode in place in the insulator, putting a glass sealing material adjacent the 35 electrode, placing a conductor adjacent the sealing material, and heating the loose assembly to mature the glaze and soften the sealing material.

2. In spark plug manufacture, the steps of coating an insulator with a glaze forming material, said'insulator having a bore therethrough. placing an electrode in the bore of the insulator at the firing end of the bore, placing a sealing material in the bore adjacent the electrode, placing a conductor in the bore adjacent the sealing material, heating the loose assembly to mature the glaze and soften the sealing material, and

applying pressure to the conductor to force it and the sealing material into place.

3. The method of claim 2, in which the assembly is heated to a temperature in the vicinity of Orton cone O6.

4. The method of claim 2, in which the loose assembly is heated to a temperature in the range from Orton cone 010 to O2.

5. In spark plug manufacture, the steps of coating a ceramic insulator requiring a high firing temperature with a glaze forming material, placing an electrode in the insulator, placing a sealing material adjacent the electrode, placing a conductor adjacent the sealing material, and heatingthe assembly to mature the glaze and soften the sealing material.

6. The process of claim 5, in which the assembly is heated to a temperature of a much lower magnitude than the temperature necessary to fire the insulator.

7. A spark plug having a ceramic insulator of a material matured at a firing temperature in the range of Orton cone 23 to 34, and a glaze coating on said insulator which matures at a temperature in the range of Orton cone 010 to O2.

8. In a spark plug, an insulator having a high proportion of sintered oxides, a glaze coating having a maturing temperature within the range of Orton cone 010 to 02 on at least a portion of said insulator, an electrode and a terminal in the insulator, and two conducting glass seals joining the electrode and terminal, the las of the seal adjacent the terminal having a greater plasticity than that of the seal adjacent the electrode.

9. The invention of claim 8, in which the glass of the seal adjacent the terminal is a lead borosilicate glass, and the glass of the seal adjacent the electrode is a borosilicate glass.

10. A spark plug having an insulator which has been fired at a temperature in the range of Orton cone 23 to 34, an electrode and terminal in said insulator, a glass seal joining said electrode and terminal, and a glaze coating on at least a portion of the insulator, said glaze coating having been matured at a temperature in the range of Orton cone 010 to O2.

11. In a spark plug, an insulator having a high proportion of sintered oxides, and a glaze coating on at least a portion of said insulator, said glaze coating having been matured at a temperature within the range of Orton cone 010 to O2.

12. In spark plug manufacture, the steps of coating a ceramic insulator which has been fired at a high temperature with a. glaze forming material, placing an electrode in the fired insulator, placing a sealing material adjacent the electrode, placing a conductor adjacent the sealing material, heating the assembly to mature the glaze and soften the sealing material simultaneously and applying pressure to the conductor to force it and the sealing material into place.

13. In spark plug manufacture, the steps of coating a ceramic insulator having a high proportion of sintered oxides with a glaze forming material having a maturing temperature in the range of Orton cone 010 to O2, placing an elec trode in the insulator, placing a glass sealing material that becomes soft and plastic at the said glaze maturing temperature adjacent the electrode, placing a conductor adjacent the sealing material, and heating the assembly at a temperature within the range of Orton cone 010 to O2 to mature the glaze and soften the sealing material.

TAINE G. MCDOUGAL. 

